This article uses couple-level data from the Fragile Families and Child Wellbeing Study (N= 2,263) to investigate factors associated with unmarried parents’ expectations about marriage and the association between their expectations and subsequent union transitions. In most couples, both partners expect to marry, and their shared expectations are the strongest predictor of marriage and separation following their child's birth. Although men's expectations are somewhat more consequential for union transitions, marriage and relationship stability are more likely when at least one parent expects to marry. Factors such as children from previous relationships, distrust, conflict, and shared activities are also associated with union transitions. Findings about how expectations and other factors relate to marriage and separation may inform new marriage promotion initiatives.